Near-infrared photometry of carbon stars in the Sagittarius dwarf irregular galaxy and DDO 210

Aims.We investigate the intermediate-age asymptotic giant branch stellar population of two Local Group dwarf irregular galaxies to characterize their carbon star population in near-infrared (IR). Methods.Our work is based on near-IR photometry complemented with optical ground based and Hubble Space Telescope (HST) photometry. Near-IR photometry is based on our and archival J and Ks-band images from SOFI near-IR array of the ESO New Technology Telescope (NTT). Optical photometry for DDO 210 is from the EMMI optical imager of ESO NTT, while the SagDIG optical data come from Momany et al. (2005, A&A, 439, 111). Results.We show that near-IR photometry is a very powerful tool for carbon star detection. We recovered two out of three previously-known carbon stars in DDO 210 and discovered six additional objects in this galaxy which have optical and near-IR colors consistent with carbon giants. This brings the total number of bona fide C-star candidates in DDO 210 to nine. However, to confirm the nature of these objects additional higher spatial resolution imaging or spectroscopic data are necessary. We detected a large population of C-star candidates in SagDIG, 18 of which were previously identified in Demers & Battinelli (2002, AJ, 123, 238) and Cook (1987, Ph.D. Thesis), and six new bona fide carbon stars. We present their optical and near-IR colors and use their luminosity function to put constraints on the star formation history (SFH) in this dwarf irregular galaxy